This invention relates to the problems of filtration of paper pulp and more particularly paper pulp that is obtained from the pulping of old paper.
In paper pulp technology, it is known to use various filtration devices either to eliminate foreign elements or to classify paper pulp fibers according to their length or else to increase the paper pulp concentration by partial elimination of water.
Among the known filtration devices, filters are present that comprise a rotor and a stator, one of the two, called a screen below, performing die function of filtering and comprising perforations such as slots or holes, the other, called backwashing element below, performing the function of backwashing and comprising means intended to generate pressure pulses to prevent the clogging of the perforations. The screen can therefore be stationary or rotating, and, conversely, the backwashing element is rotating with a stationary screen aid stationary with a rotating sieve. In addition, the liquid can either flow through the sieve from the outside to the inside (toward the axis), so-called centripetal operation, or from the inside to the outside, so-called centrifugal operation.
In this type of device, the pulp reaches one end of the cylindrical screen, flows into the space between the cylindrical sieve and die backwashing filter, and comes out at the other end.
As the pulp advances into the filter, it becomes increasingly thick.
This gradual thickening of the pulp produces several effects: a pulp flocculation effect and a sieve clogging effect.
Even if the pulp that is admitted has previously been well deflocculated, there is the danger of flocks appearing during its advance; the greater the flocculation, the more significant the clogging.
If the backwashing filter rotates, it has a tendency to rotate the pulp with it. If the backwashing element is stationary, the screen has a tendency to rotate the pulp. In the two cases, as the backwashing effect by the pressure pulses that are caused by the backwashing element is directly linked to the speed deviation between the backwashing filter and the pulp, the more the pulp advances to the end of the screen, the less effective the backwashing is, while at the same time the screen is more prone to clogging.
In addition, with a fine-slot screen, the long fibers have more trouble passing than the short fibers, and therefore the proportion of long fibers relative to the short fibers increases with the advance of the pulp, which accelerates both the flocculation process and that of the clogging.
This accumulation of phenomena brings about a gradual reduction in production per unit of surface area of the screen as the pulp advances in the filter.
Many patents have had as their object to combat this gradual reduction of the productivity of the filter by intervening, during the travel along the screen, so as to create strong deflocculation zones and/or zones for greatly slowing the pulp.
In particular, from 1968 in her French Patent 1,539,816, the applicant proposed using obstacles whose purpose is to break the component parallel to the surface of the sieve from the speed of the liquid.
U.S. Pat. No. 4,383,918 proposed using obstacles that create turbulence between the screen and the rotor.
Patent FR 2,613,390, also in the name of the applicant, proposed separating the screen into several portions and introducing water for dilution between the portions of the sieve.
English Patent GB 2,222,967 proposed installing two sieves that are separated by stationary flanges placed between the surface of the screen and the rotor.
All of these devices have gradually improved the operation of filtration devices that consist of a sieve and a backwashing filter bait in a way that is still inadequate.
According to this invention:
The zone where the speed deviation increases can be one or more baffles;
The zones for deflocculation and increasing the speed deviation can be combined.
This invention relates to a filtration device or purifier of the type that comprises a screen of cylindrical shape and a coaxial rotor that is equipped with blades that produce pressure fluctuations to prevent the clogging of said screen; whereby said screen and said rotor comprise at least two portions of approximately equivalent diameter, whereby these portions are separated from one another by means whose object is to break the component from the speed of the liquid, which is parallel to the surface of the screen, and to create turbulence characterized by the fact that said means are baffles that consist of, on the one hand, cavities that are placed between the portions of the screen, whereby the bottom of said cavities is optionally farther from the rotor than the surface of the screen; and, on the other hand, annular deflectors that are carried by the rotor opposite each cavity; such that it is necessary for the majority of the liquid flow to pass through the cavities.
This invention can also comprise all or part of the following arrangements, taken together or separately:
a The screen consists of a stack of rings,
b The screen consists of a grid that is obtained by juxtaposing bars that are either parallel to the axis of rotation of the rotor or perpendicular or inclined,
c The screen consists of a juxtaposition of grids as described in the preceding paragraph, whereby these grids are separated by rings,
d The screen consists of a cylindrical sheet-metal plate that is perforated with holes or slots,
e The screen consists of a stack of cylindrical pieces of sheet metal that are perforated with holes or slots,
f The cavities are parallelepipedic hollow volumes,
g The cavities are cylindrical hollow volumes,
h Some cavities can be connected at the bottom to a dilution intake which can be ensured by water or pulp that is less concentrated than the filtered pulp.
The purpose of this invention is also the use of this device with regulating means that make it possible:
a) either to adjust the rotor speed based on the pressure differential of the pulp between the inlet of the device and at least one of the accepted outlets;
b) or to adjust the rotor speed based on the flow rate or rates in at least one of the outlets for accepted products;
c) or to adjust the flow rate of the output for rejects from the device based on the accepted flow rates and dilution flow rates that are measured to regulate the rate of rejects from one or more portions of the screen;
d) or by combining two or three of these regulating means.
The purpose of this invention is also the use of means that are described above for the filtration of the paper pulp.